Oil for transformers and the like



M. CLARK OIL FOR TRANSFORMER AND THE LIKE Filed April 23, 1932 0/L CONTAINS IVAFH TH YL AIM/IVE Inventor Frank Clark,

b g v HisAttor'ne g.

DA Y8 umJ4 Patented Jan. 15, 1935 PATENT OFFICE 1,988,299 on. roa musrommns AND ma Linn Frank M. Clark, manna, um, inimito General Electric Company, a corporation of New York Application April as, 1932, Serlal No. 601,14:

Claims. (o1. ass- 1) The present invention relates to the suppression oi' sludge formation in petroleum products, such as the oil content of electrical devices, such as transformers, wherein mineral oil is used for 5 cooling and insulating purposes. I

A semi-solid precipitate known as sludge is formed inmine'ral oil during transformer service when such oil is heated without special precautions to an elevated temperature. The opcrating temperatures of the oil vary with the character and loading of transformers but ordinarily they are well below 150 (2., as observed at the hottest point in a transformer. Sludge, when formed, clogs the cooling ducts and lowers the dielectric qualities of the oil. It consists largely of tarry or asphaltic polymerization and esterification products. Although slud'ged transformer oil may be reclaimed by the use of acid or fullers earth, or both, and used over again Y when mixed with new oil, the net effect of the sludging of transformer oil is an economic loss and a constant threat to the continued safe operation of the apparatus.

While it is possible to employ in a transformer, or the like, a highly refined mineral oil in which sludge will not be formed under operating conditions, such oil not only is expensive but it has been found to be subject to an even more objectionable acid formation. In the highly refined oil. which has been freed of unsaturated hydrocarbons, corrosive acids of the acetic acid type are formed. Theseattack actively metal parts of the device, and seriously afi'ect the mechanical and electrical properties of the insulation immersed therein.

In some respects an unrefined oil distillate, that is, a chosen fraction of crude oil distillation, is better suited for dielectric purposes than a highly refined oil. Its market price is only about a tenth the price of highly refined oil. How: ever, it has been found that the sludge forming characteristics-of such oil may vary from shipment to shipment. Its use thus entails a serious laboratory control expense. In any event, as elsewhere more fully explained the unrefined distillate forms sludge more readily than a refined 011. Therefore, it has been the practice to employ for transformer purposes a special partially refined oil in which a compromise is made between various qualities including sludging and acid formation.

Partially refined oil, that is, hydrocarbon distillates from which unsaturated components have been partially removed (by treatment with 5 ordinary sulphuric acid as distinguished from the filming sulphuric acid employed for producing a highly refined oil) costs about one-half as much as highly refined oil.

As a result of a long series of experiments I 10 have discovered that when there is associated with unrefined oil a small amount of a naphthyl amine compound, ordinarily less than one per cent by weight, that sludsing is substantially suppressed. In accordance with my invention 15 sludging is not only suppressed but the advantages of the low cost unrefined distillate are secured without difilculties from the excessive acid formation, characterizing the highly refined product. 20

The accompanying drawing, Fig. 1, is .a side elevation of a transformer which is shown as being illustrative of the most useful commercial application of my invention; and Fig. 2 is a graph showing by accelerated sludging tests the 5 Q beneficial effect of my invention on mineral oil.

The beneficial effect of such amine in the oil appears to be due to both a retardation of the polymerization and esterification reactions whereby sludge products are formed and a peptizing effect whereby such products as may be formed by such action are kept in solution and hence do not interfere with the function of the oil in the transformer or the like.

The benefits of my invention also may be obtained to a substantial extent with partly refined hydrocarbon oils which may be employed in combination with such naphthyl amine fora period of service which is much longer than the service life of partly refined oil unassociated 0 with such amine.

In carrying out my invention a primary or secondary naphthyl amine, such for example as alpha or beta naphthyl amine, in desired amount is dissolved in a convenient quantity of hydrocarbonoil which then is added to the main body of oil in which suppression of sludgins is desired. I prefer to employ phenyl alpha naphthyl amine. I prefer to add to the oil about onehalf of one per cent by weight of the amine. I

prefer also to employ unrefined distillates having a flash point not less than 125 to 130 C. and a viscosity of approximately 50 to 60 seconds Saybolt Universal at 318 C. Unrefined distillates may have iodine values as high as 18 to 2i). Partially refined oil, such as commonly used in transformers has iodine values of about 7 to 8. Highlyrefined oil has a zero iodine value. I prefer to use an oil having an iodine value not higher than 15 to 18, no free sulphur, less than 5/10 per cent fixed sulphur; although as high as may be used.

One example of a suitable distillate has the following properties:

Flash point 135 C. Burn point 150 C. Specific gravity 15.5 .900 Viscosity 37.8 58 seconds Inorg. content; Acidity less than 1.0 mg. NaOI-I per gram of oil Iodine number 18 Fixed sulphur; .5 percent Free sulphur 0 Referring to Fig. l of the drawing, the transformer which is illustrated comprises a, tank 1, containing an iron core 2, and electrical windings 3, the terminals of which are indicated at 4, 4'. The core and windings are immersed in oil as indicated at 5. As indicated by legend on the drawing the oil 5 contains dissolved therein a naphthyl amine.

The benefits of my invention have been demonstrated under service conditions. In my tests the effect of a great many different materials on such oil has been investigated. As an.oilfilled type transformer contains a large quantity of oil and as sludge may require several years to make its appearance in a transformer which operates at a; temperature only moderately above ambient temperatures, it would require a long time ordinarily to test the efiicacy of a material as an anti-sludging reagent in mineral oil under service conditions but tests' have been devised whereby the formation of sludge in oil mixes is accelerated. In accordance with one such test small oil samples (and any substance associated therewith) are heated at 110 C. in contact with copper (which accelcrates sludging) and then such sludge as may be formed is centrifugally separated from the oil in suitable test tubes. Its volume may be observed. In accordance with an even more rapid test small samples of oil are subjected at a temperature of 140 C. in a closed container (bomb) for 24 hours to the effect of pure oxygen at a pressure of 250 pounds to the square inch. The sludge formed under these conditions is centrifugally separated and observed. It is usually expressed in percent by comparison with weight of the original oil sample.

In Fig. 2 I have shown graphically the result of a series of tests made with mineral oil containing phenyl alpha naphthyl amine which was subjected to normal operating conditions in a transformer containing over four hundred gallons of oil. The results obtained were found to check with preliminary tests. They were checked again with bomb tests involving treatment with oxygen under'pressure as described above, and again the results were found to be in agreement.

Referring to Fig. 2, the length of time oil samples were subjected to accelerated sludging conditions are plotted as abscissa ind y f 24 hours each. The percent of sludge formation noted after centrifuging are plotted as ordinates. Curve a" shows the results obtained with oil samples which were the same as the oil which was introduced into a transformer, namely ordinary transformer oil which had been partly refined-by ordinary sulphuric acid and to which was added by weight about percent of phenyl alpha naphthyl amine. After forty days of heating at 110 C. in contact with copper only 1% of sludge could be observed in a test sample as indicated by curve a. Curve 1) shows the effect of a similartest on similar oil samples taken from the transformer after three months service. One percent of sludge could .be observed after thirty-five days heating of the sample. Curve 0 shows the results obtained with small samples taken from the transformer after six months service. One percent of sludge appears in a sample after about thirty days heating. Curve 01 shows that thereafter little change tookplace as after one years service only slightly more sludge could be noted in a test sample heated for thirty days. Curve e shows check tests performed on new commercial transformer oil (unassociated with naphthyl amine or other retarder). About one percent of sludge appeared in such new oil after twenty-five days heating under the same conditions. Curve -f shows the result of the same accelerated sludge test on an unrefined distillate. very steeply and shows 1.2 percent of sludge in a test sample in twelve to thirteen days. Un"

refined distillate containing a naphthyl amine shows itself by test to be substantially equal to a partially refined transformer oil (curve e") or even better in some cases. No separate curve for such a combination has been shown as it tends nearly to coincide with other curves and would thus tend to confuse the graph.

Bomb test experiments as above described have not'been plotted but led to the same conclusion. New transformer oil containing phenyl alpha naphthyl amine gave .05% sludge by weight. After three months service the weight of sludge obtained by bomb test was unchanged. After six months service .06% of sludge was observed in a test sample. After one years service .05% of sludge was observed. In a sample of new transformer oil .06% of sludge was observed by this test. Similar favorable results have been observed with other naphthyl amines. These results are all the more surprising because of the fact that some amines have been found to be accelerators of sludging.

What I claim as new and desire to secure by Letters Patent of the United States is:

This curve rises 1. A petroleum composition suitable for use at amine.

2. A petroleum composition comprising potentially sludging mineral oil and phenyl alpha naphthyl amine associated therewith, said oil so modified being resistant fora materially longer time to the formation of sludge at elevated temperature than when unassociated with such amine.

3. A composition suitable for use as an in- .sulating and cooling dielectric material in electric devices comprising mineral oil having an iodine value within the limits of about '1 to 20 and tending by itself when heated during service to form insoluble, semi-solid products, and about one-halfof one per cent of phenyl alpha naphthyl amine, whereby the formation of such semisolid products is materially retarded.

4. The process of preventing sludging in mineral oil containing sludge-forming constituents which comprises maintaining a naphthyl' amine in association with such oil during use under conditions which in the absence of such amine would form sludge therein.

5. A composition comprising the combination of an unrefined mineral oil and about one-half of one per cent by weight thereof of phenyl alpha naphthylamine.

FRANK M. CLARK. 

